One aspect of semiconductor wafer processing includes making buried contacts to field effect transistor gate lines. A conventional gate line typically comprises a gate dielectric layer and a conductively doped polysilicon layer (typically n+doped) and an overlying silicide layer (i.e., WSi.sub.x). These gates are typically fabricated by deposition or provision of these three layers over a semiconductor substrate, followed by collectively patterning these layers with photoresist to form the desired gate outlines. An insulative capping material might also be provided over the silicide layer prior to patterning to form the conductive portions of the gate line. Transistor gates might also be fabricated using damascene methods, and also above or below a thin film semiconductor layer such as in fabrication of semiconductor-on-insulator circuitry which might be top or bottom gated.
A thick insulating layer, such as borophosphosilicate glass, is typically provided over the resultant transistor and provided with an upper planar surface. Contact openings can then be etched through the insulating layer to the outer conductive portion of the transistor gates, as well as to other substrate areas. The openings are filled with conductive plugging material. Metal or conductively doped semiconductive material, such as polysilicon, are example materials.
In certain applications, it may be desirable that the conductive plugging material be a semiconductive material having opposite type conductivity enhancing dopant impurity as compared to the conductivity type impurity within the semiconductive material of the gate. For example where the gate is heavily doped to achieve conductivity with n-type material, in some applications it might be desirable to provide a conductively doped contact plug to that gate with p-type material. Unfortunately, the different dopant types can easily cross-diffuse relative to one another through the silicide which can lead to no conductive connection. One prior art solution to avoiding this diffusion is to initially line the contact opening with a very thin layer of an electrically conductive diffusion barrier material, such as TiN. Subsequently, the remaining portion of the opening is filled with conductively doped polysilicon to provide the desired electrical connection with the transistor gate.